In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size ≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA≧0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 laser lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography. Efforts have been made for the early introduction of ArF immersion lithography (see Proc. SPIE, Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724, 2003).
Several problems arise when a resist film is exposed in the presence of water. For example, the acid once generated from a photoacid generator and a basic compound added to the resist can be partially leached in water. As a result, pattern profile changes and pattern collapse can occur. It is also pointed out that water droplets remaining on the resist film, though in a minute volume, can penetrate into the resist film to generate defects.
These drawbacks of the ArF immersion lithography may be overcome by providing a protective coating between the resist film and water to prevent resist components from being leached out and water from penetrating into the resist film (see the 2nd Immersion Workshop, Resist and Cover Material Investigation for Immersion Lithography, 2003).
With respect to the protective coating on the photoresist film, a typical antireflective coating on resist (ARCOR) process is disclosed in JP-A 62-62520, JP-A 62-62521, and JP-A 60-38821. The ARCs are made of fluorinated compounds having a low refractive index, such as perfluoroalkyl polyethers and perfluoroalkyl amines. Since these fluorinated compounds are less compatible with organic substances, fluorocarbon solvents are used in coating and stripping of protective coatings, raising environmental and cost issues.
Other resist protective coating materials under investigation include water-soluble or alkali-soluble materials. See, for example, JP-A 6-273926, Japanese Patent No. 2803549, and J. Photopolymer Sci. and Technol., Vol. 18, No. 5, p 615, 2005. Since the alkali-soluble resist protective coating material is strippable with an alkaline developer, it eliminates a need for an extra stripping unit and offers a great cost saving. From this standpoint, great efforts have been devoted to develop water-insoluble resist protective coating materials, for example, resins having alkali-soluble groups such as fluorinated alcohol, carboxyl or sulfo groups on side chains. See WO 2005/42453, WO 2005/69676, JP-A 2005-264131, JP-A 2006-133716, and JP-A 2006-91798.
Required of the resist protective coating materials are not only the ability to prevent the generated acid and basic compound in the photoresist film from being leached out in water and to prevent water from penetrating into the resist film, but also such properties as water repellency and water sliding property. Of these properties, water repellency is improved by introducing fluorine into the resin and water sliding property is improved by combining water repellent groups of different species to form a micro-domain structure, as reported, for example, in XXIV FATIPEC Congress Book, Vol. B, p 15 (1997) and Progress in Organic Coatings, 31, p 97 (1997).
One exemplary polymer exhibiting high water sliding property and water repellency is a fluorinated ring-closing polymerization polymer having hexafluoroalcohol pendants. It is reported in Proc. SPIE, Vol. 6519, p 651905 (2007) that this polymer is further improved in water sliding property by protecting hydroxyl groups on its side chains with acid labile groups.
Although the introduction of fluorine into resins is effective not only for improving water repellency, but also for improving water sliding properties as demonstrated by sliding angle, receding contact angle or the like, excessive introduction of fluorine results in resins with a greater surface contact angle following alkaline development. In the current technology, those defects so called “blob defects” that occur on the resist film surface (especially in the unexposed area) after development are regarded problematic. A tendency is known that a resist film having higher water repellency suffers from more blob defects. Accordingly, introducing extra fluorine into resins for the purpose of enhancing water repellency and water sliding property increases a likelihood of blob defects occurring.
It is believed that blob defects are caused by water droplets remaining on the resist film surface after development. The internal energy of a water droplet on a resist film increases in the spin drying step and reaches the maximum when the water droplet completely leaves the resist film surface. At the same time as the water droplet leaves the resist film surface, the resist film surface is damaged by that energy, which is observable as blob defects.
The internal energy of a water droplet on a resist film is higher as the surface becomes more water repellent. When a protective coating with higher water repellency is disposed on a resist film, the resist surface has a greater contact angle due to intermixing between the resist film and the protective coating, increasing a likelihood of blob defects occurring. This indicates that for the purpose of suppressing the occurrence of blob defects, the surface contact angle after development must be reduced to mitigate the internal energy of a water droplet.
Application of a more hydrophilic resist protective coating may be effective for reducing the surface contact angle after development. However, such a protective coating provides a smaller receding contact angle, which interferes with high-speed scanning and allows water droplets to remain after scanning, giving rise to defects known as water marks. A resist protective coating having carboxyl or sulfo groups is proposed in U.S. Pat. No. 7,455,952 (JP-A 2006-91798). Since both carboxyl and sulfo groups are fully hydrophilic, water repellency and water sliding property worsen.
It is then proposed to form a protective coating from a blend of a first polymer having sulfo groups and a second polymer having highly water repellent hexafluoroalcohol groups such that the second polymer having hexafluoroalcohol groups is segregated at the surface of the protective coating and the first polymer having sulfo groups is segregated at the interface with the underlying resist. See 4th Immersion Symposium RE-04 New Materials for surface energy control of 193 nm photoresists, Dan Sander et al. Although this protective coating is effective in reducing blob defects, the resist pattern suffers from film slimming after development because sulfo groups bind with an amine component in the resist so that the amine component becomes depleted near the resist surface. There exists a desire for a protective coating which prevents film slimming in order to produce a rectangular profile pattern and renders more hydrophilic the resist surface after development in order to inhibit blob defects.
The resist protective coating materials discussed above are needed not only in the ArF immersion lithography, but also in the electron beam (EB) lithography. When EB lithography is performed for mask image writing, it is pointed out that the resist changes its sensitivity due to evaporation of the acid generated during image writing, evaporation of vinyl ether produced by deprotection of acetal protective groups, or the like, as discussed in JP-A 2002-99090. It is then proposed to suppress resist sensitivity variation by applying a protective coating material to form a barrier film on top of a resist film.
As means for preventing resist components from being leached out and water from penetrating into the resist film without a need for a protective coating material, it is proposed in JP-A 2006-48029, JP-A 2006-309245, and JP-A 2007-187887 to add an alkali-soluble, hydrophobic, high-molecular-weight compound as a surfactant to the resist material. This method achieves equivalent effects to the use of protective coating material because the hydrophobic compound is segregated at the resist surface during resist film formation. Additionally, this method is economically advantageous over the use of a protective film because steps of forming and stripping the protective film are unnecessary.
It is believed that independent of whether the alkali-soluble surfactant or the resist protective coating material is used, water droplets remaining on the resist film or protective film after scanning cause failure (or defects) in pattern formation. The ArF immersion lithography systems commercially available at the present are designed such that exposure is carried out by scanning the wafer-mounted stage at a speed of 300 to 550 mm/sec while water is partly held between the projection lens and the wafer. In the event of such high-speed scanning, unless the performance of the resist or protective film is sufficient, water cannot be held between the projection lens and the wafer, and water droplets are left on the surface of the resist film or protective film after scanning. Such residual droplets can cause defects to the pattern.
To eliminate defects owing to residual droplets, it is necessary to improve the flow or mobility of water (hereinafter, water sliding property) on the relevant coating film and the water repellency of the film. It is reported effective to increase the receding contact angle of the resist or protective film with water. See 2nd International Symposium on Immersion Lithography, 12-15 Sep., 2005, Defectivity data taken with a full-field immersion exposure tool, Nakano et al.
For improving the water repellency of a coating film, introduction of fluorine into a base resin is effective. For improving water sliding property, combining water-repellent groups of different species to form a microdomain structure is effective. See XXIV FATIPEC Congress Book, Vol. B, p 15 (1997) and Progress in Organic Coatings, 31, p 97 (1997). According to these reports, when a water molecule interacts with methyl and trifluoromethyl groups, it orients via its oxygen and hydrogen atoms, and the orientation distance between water and methyl is longer. Thus a resin having not only water repellent fluorinated units introduced, but also both fluoroalkyl and alkyl groups incorporated is improved in water sliding property because of a longer orientation distance of water.
One exemplary material known to have excellent water sliding property and water repellency is a copolymer of α-trifluoromethylacrylate and norbornene derivative (Proc. SPIE, Vol. 4690, p 18, 2002). While this polymer was originally developed as a highly transparent resin for F2 (157 nm) lithography resist materials, it is characterized by a regular arrangement of molecules of water repellent α-trifluoromethylacrylate and norbornene derivative in a ratio of 2:1. This characteristic arrangement increases the orientation distance of water relative to the resin and improves water sliding property. In fact, when this polymer is used as the base polymer in a protective coating for immersion lithography, water sliding property is drastically improved, as described in JP-A 2007-140446 or US 20070122736.
Another example of the highly water repellent/water sliding performance material is a fluorinated ring-closing polymerization polymer having hexafluoroalcohol groups on side chains. This polymer is further improved in water sliding property by protecting hydroxyl groups on side chains with acid labile groups, as reported in Proc. SPIE. Vol. 6519, p 651905 (2007).
A material having good water sliding property performance is required not only from the standpoint of defects, but also from the standpoint of productivity. The immersion lithography needs higher throughputs than ever. For improved productivity, the exposure time must be reduced, which in turn requires high-speed scanning operation of the stage. In order to move the stage at a high speed while holding water beneath the lens, it is desired to have a resist material or resist protective film having higher water sliding property performance.
The highly water repellent/water sliding performance materials discussed above are expected to be applied not only to the ArF immersion lithography, but also to the resist material for mask blanks. Resist materials for mask blanks suffer from problems including a change of sensitivity during long-term exposure in vacuum and long-term stability after coating. With respect to the control of sensitivity changes in vacuum, an improvement is made by a combination of acid labile groups of acetal and tertiary ester types (U.S. Pat. No. 6,869,744). It is believed that after coating of a resist material, an amine component is adsorbed to the resist film surface whereby the resist varies its sensitivity or profile. A method of modifying the surface of a resist film for preventing adsorption of an amine component to the resist film has been devised.